A thermal anemometer is a device used to measure liquid and gas flow rates. Thermal convection of heat from an actuated sensor to the ambient environment is the primary transduction mechanism. That is, as heat is conducted away from the device, a thermo-resistive change takes place in the transducer which causes the electrical state of the device to change. This change in electrical state is measured either directly or indirectly through, for example, a Wheatstone bridge.
The thermo-resistive active area of the sensor is typically a hot wire or hot film of metal with a known temperature dependent resistance. The active area must be heated to some temperature above the ambient temperature, or else thermal convection does not occur. Current driven through the thermo-resistor serves to heat the active area according to Joule's First Law.
Thermal anemometers are normally operated in one of three modes: constant current, constant voltage, or constant temperature. In each case, the current, voltage, or temperature of the device is maintained as the flow rate changes. The change in device temperature, as already explained, causes a concomitant change in resistance. Constant current circuits are preferable when the amount of current needs to be precisely controlled to prevent adverse effects, such as overheating, which can lead to premature device failure.
Because the conductors are generally small and thermal conduction noise minimal, hot wire anemometers are often considered the preferred method for flow rate measurement. However, hot wire anemometers tend to be expensive to fabricate and fragile. For this reason, hot film anemometers are often preferred.
In hot film anemometry, the active layer is a thin metal film (such as platinum) or semiconductor (such as silicon) supported by a flat insulating layer. Many hot metal film anemometers are open bridge configurations where the active film is minimally supported by a thin membrane over a cavity. While the thin membrane (e.g., silicon dioxide) reduces the thermal conduction loss pathways, it also leads to device failure in extreme temperature and flow conditions due to uncontrolled stress and strain.
In prior art, U.S. Pat. No. 5,310,449, U.S. Pat. No. 5,231,877, and U.S. Pat. No. 4,930,347, all to Henderson, methods of fabrication for current driven semiconductor film microanemometers are described. The silicon active element responds dynamically to changes in ambient temperature due to its well known intrinsic semiconducting properties. Devices from the '449, '877, and '347 patents to Henderson, were susceptible to failure due to thin nitride support bridges which carried the metal contacts from the bulk to the sensor. Although these devices showed excellent sensitivity and response time, their tendency to fail prematurely in harsh environments limited their application.
U.S. Pat. No. 6,032,527 to Genova, the entire contents of which are incorporated herein by reference, distinguished itself from such earlier art by devising a novel sensor support scheme. The bonded wafer approach proposed in the '527 patent included a slight overlap between the sensing and support layers that maximized ruggedness without significantly affecting sensitivity. The amount of overlap and thus ruggedness were predetermined by the intended end application of the sensor. The device according to Genova also included through-wafer interconnects instead of wire bonds. Top surface wire bonds used in the prior art were routes for failure in high flow applications where the bonds could be sheared or acted to significantly impede the flow. Not only did the through-wafer electrical interconnects devised by Genova reduce the likelihood of interconnect failure, but they also facilitated back side contact to the packaging, protected the interconnects from the ambient flowstream, and allowed for simplified front side passivation before dicing.
The active element of the hot film anemometer presented in this invention is a thin mesa of silicon supported by both a silicon dioxide membrane and silicon substrate, similar to the device in the '527 patent. Recent advances, however, in microfabrication, dry film resists, spin-on glass, engineered glasses and packaging techniques have allowed for numerous improvements in the art.